The Radiative Transfer Approach in Electromagnetic Imaging

Abstract

In the past, the radiative transfer theory has been largely concerned with the propagation of scalar intensities. In recent years, however, there has been an increasing interest in the propagation of complete arbitrarily polarized electromagnetic waves. We first present the general formulation of the complete radiative transfer equation using the Stokes1 vectors, including the extinction matrix and the Mueller scattering matrix. The coherent and incoherent Stokesf vectors are defined corresponding to the coherent and incoherent field. The coherent Stokes1 vector is shown to have a depolarization effect. Solutions of the vector radiative transfer equation require Fourier decomposition to account for the polarization effects. Solutions for linearly polarized and circularly polarized waves in spherical particles are presented, showing the angular dependence and the degree of polarization. It is indicated that even in spherical particles, depolarization occurs due to multiple scattering. Discussions are also presented on the propagation of Stokes’ vectors in nonspherical particles. Depolarization effects and the deterioration of images are discussed in terms of the solutions of the radiative transfer equation. The limitations of the radiative transfer equation and its relation to multiple scattering theory are also discussed. It is shown that when the density is high, the radiative transfer theory becomes increasingly approximate.